Hi I have a SW Black Diamond 180mm Mak - for lunar & planetary - thinking f/15 might be OK, however f/10 is a better choice. At f/15 the 180mm Mak can only span medium to high power, low powers are just out of the question. My favorite eyepieces on this scope are a Vixen LV50mm and the Vixen LVW13mm. An 8mm is also useful if the seeing permits.
On mine the 50mm finder fits perfectly and manages to stay aligned between setups, ie goes back together quite well.
On the Black Diamond 180mm the rear adapter is not the same (it is significantly bigger) as the back on the 8" Meade/Celestron scopes, and mine can easily fill the field of view in a Vixen LV50mm or the Masuyama 45mm (this is about a 45 arc minute field of view) with a nice field sharp to the edge. It can't quite fill the TMB40mm (outer 5mm is vignetted).
I've come to the conclusion f/7 is probably nirvana as it allows a range of eyepieces spanning 10:1 between highest and lowest power. As an alternative I'd also suggest this 127mm f/7.5 APO:
http://explorescientific.com/telesco...mm_ed_apo.html